Noise cancelling self mixing doppler radar

ABSTRACT

A noise cancelling self-mixing Doppler type microwave motion detector with greater detecting range and/or lower false alarm rate than prior art self-mixing techniques using equal radiated power is described. The device includes a three terminal microwave oscillator/mixer and a wave guide launched dielectric antenna in a single triaxial structure and a dual input signal processor that provides improved signal to noise ratio as a result of effective signal enhancement and noise cancellation. Separate, out of phase, Doppler output signals are derived from both the emitter and the base of the oscillator/mixer transistor and processed by a balanced amplifier which rejects in-phase noise components.

United States Patent Friedman et al.

NOISE CANCELLING SELF MIXING DOPPLER RADAR Inventors: Robert L.Friedman, 51-25 Van Kleeck St., Elmhurst, NJ. 11373;

Zeev Lieser, 1048 Coolidge Rd., Elizabeth, NJ. 07208 Filed: Aug. 31,1972 Appl. No.: 285,251

References Cited UNITED STATES PATENTS Gupta 343/8 PrimaryExaminer-Benjamin A. Borchelt Assistant-Examiner-G. E. Montone Attorney,Agent, or Firm-Charles E. Temko [57] ABSTRACT A noise cancellingself-mixing Doppler type microwave motion detector with greaterdetecting range and/or lower false alarm rate than prior art self-mixingtechniques using equal radiated power is described. The device includesa three terminal microwave oscillator/mixer and a wave guide launcheddielectric antenna in a single triaxial structure and a dual inputsignal processor that provides improved signal to noise ratio as aresult of effective signal enhancement and noise cancellation. Separate,out of phase, Doppler output signals are derived from both the emitterand the base of the oscillator/mixer transistor and processed by abalanced amplifier which rejects in-phase noise components.

3 Claims, 2 Drawing Figures PAIENIEBJANZZIBH SHEET 1 ll? 2 PAIENIEDJAN22 I974 SHEET 2 0F 2 BRIEF DESCRIPTION OF THE PRIOR ART The state ofthe art in high performance microwave Doppler motion detectors generallyutilizes a two or three terminal conventional oscillator, separateradiating and receiving antennas, or a single antenna with a circulator,mixer diode and processing amplifier circuitry arranged into a homodyneor zero IF system.

Economy, lower performance systems utilize the oscillator as aself-mixer. The non-linearity of the oscillator device itself performsthe mixing function and allows a single radiating/receiving antenna tobe used. Sensitivity of the system in this mode is inferior to theconventional method described above, partially due to the higher noisefigure of the oscillating detector which contains greater l/f or flickernoise as compared to a high quality Schottky diode, and also because ofthe AM noise content of the oscillator which is no longer isolated fromthe mixer by the injection decoupling of the higher performance system.The derived Doppler output signal is thus superimposed upon these noisecomponents which degrade the signal to noise ratio and so limit theoperational distance capability.

BRIEF DESCRIPTION OF THE PRESENT INVENTION Briefly stated, the inventioncontemplates a low-loss, high dielectric antenna structure whichsimultaneously serves as a transmitting antenna for a coupledoscillator, CW or pulsed, and as a receiving antenna coupled into thefield of said oscillator. Target motion within the range of thedisclosed embodiment causes an exhibiting of the Doppler effect suchthat the reflected and returned signal is shifted in frequency from thefundamental frequency produced by the oscillator and the coupling of thereceived signal energy into the oscillator field results in a heterodyneaction in the oscillator device junctions.

The resultant output produced from said mixing action is availableacross the impedance presented by any element of the oscillator device,such as the base to ground and emitter to ground resistances normallyrequired for forward biasing purposes, and is at the Doppler frequencyresultant of the target motion rate and the transmit carrier frequency.By proportioning the length of the oscillator line elements of the baseand the emitter to be one quarter wave length each, at the transmitfrequency, it may be seen that a phase inversion is maintained betweenthe base and the emitter for the coupled receive signal as a result ofthe standing wave pattern on the total one half wave length line elementthus created. A balanced, push-pull output is then derived from theself-oscillator/mixer for receive signal Doppler, whereas internal noisecomponents such as l/f noise and power supply ripple develop in-phaseoutput components.

Coupling the derived Doppler and noise voltages into a balanced,push-pull amplifier results in an enhanced output for the Dopplersignal, and a cancellation of the noise components. The net result ofthis action is an improved signal to noise ratio, which allows eithergreater motion detection distance range or a lower false alarm rate forequal transmit power and receive sensitivity as compared to aconventional system not utilizing the invention.

The invention is also useful for increasing the performance of separatemixer and oscillator systems by improving the signal to noise ratiothrough the elimination of the oscillator noise power. Either a two orthree terminal oscillator may be compensated for by deriving one half ofthe balanced Doppler output from across an oscillator element and theother half of the balanced Doppler signal from an opposite polarityoutput separate diode mixer.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, to which referencewill be made in the specification, similar reference characters havebeen employed to designate corresponding parts throughout the severalviews.

FIG. 1 is a schematic wiring diagram of an embodiment of the invention.

FIG. 2 is a schematic sectional view of the antenna structure forming apart of the embodiment, coupled to and from a triaxial structureoscillator.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENT In accordance with theinvention, with reference to FIG. 1 of the drawings, the device,generally indicated by reference character 10 includes a step-downtransformer 11 powered by a 1 10 volt primary winding 12 and a secondarywinding 13 connected to a bridge rectifier 14 to produce 12 volts DC.Current flows through a surge limiting resistor 15 to a ripple filtercapacitor 16 and an integrated circuit voltage regulator 17. A resistor18 is connected to the emitter of a transistor 32, and through a feedthrough capacitor 20 through the antenna structure 21 to a microwaveoscillator transistor 22.

Referring to FIGS. 1 and 2, a coaxial transmission structure includes acollector tube 23, a base tube 24 and an outer shield 25. Feed throughcapacitors 26 and 27 are interconnected with forward biasing resistors28 and 29 for the transistor 22. A coupling capacitor 30 and variableresistor 31 form part of a network for the transistor 32, which networkincludes resistors 33, 29, 38 and 31. A coupling capacitor 35 couplessignals from transistor 32 to transistor 19 which is enabled by aforward biasing network including resistors 35a and 35b for thetransistor 19.

The common emitter resistor 18 serves the microwave oscillatortransistor 22 and the first amplifier transistor 32. A variable resistor39 provides range i.e., distance control the resistor 31 providingproper D.C. operating level for the above mentioned transistors 19 and32 Reference characters 38 and 43 designates a low pass filter resistorcapacitor network which limits operating bandwidth at llOHz.

The transistor 42 of emitter follower type interconnects with a couplingcapacitor 41 to a triggering circuit generally indicated by referencecharacter 44. This circuit includes a resistor 45, a transistor 46, adriver transistor 47, a noise threshold diode 48, resistors 49, 50 and51; coupling capacitor 52, bias resistor 53, load diode 54 and relay 55.Reference numeral 56 designates a feed back resistor, and resistor 57and capacitor 57a form a regenerative feed back RC combination for thetriggering circuit.

Referring to FIG. 2, the structural aspects of the combination coaxialtransmission structure and transmitting and receiving antenna structureare illustrated in greater detail. The effective length of the collectortube 23 is regulated by the inner end 59 of an adjustment screw 60, sothat it can be made exactly one quarter wave length (of the operatingfrequency) long. The opposite end of the collector tube is physicallymounted on the casing of the transistor 22, and the base lead 61 isconnected to the base tube 24. The emitter lead 62 extends directly intothe cavity 63 of the housing 64 which mounts a dielectric rod 65, thelength of the emitter lead to the exit point 66 at which it connectswith the feed throughcapacitor is also one quarter wave length long. Theouter shield slips into a circular mounting member 67 interconnected tothe outer surface 68 of the housing, permitting convenient assembly ordisassembly, if required. The tubes 23-25 are insulated from each otherby spacers 70 and 71 which maintain coaxial alignment.

We wish it to be understood that we do not consider the inventionlimited to the precise details of structure shown and set forth in thisspecification, for obvious modifications will occur to those skilled inthe art to which the invention pertains.

We claim:

1. In a self-mixing Doppler radar construction including a coaxialtransmission element and a rod antenna extending from a transmitting andreceiving cavity, the improvement comprising: said coaxial transmittingelement including a first outer coaxial shield, a cavityforming housing,means on a outer surface of said housing for mounting said shield atsubstantially right angles with respect thereto, a microwave oscillatortransistor, a second tube coaxially disposed within said outer shieldinterconnected to the base of said transistor, a third tube coaxiallydisposed within said second tube and interconnected to the collector ofsaid transistor; said transistor being disposed within the base of saidthird tube, and having an emitter lead extending into said cavity insaid housing.

2. Structure in accordance with claim 1, further characterized in thatsaid third tube has a length, and said dielectric cavity has a diameterequal to one quarter of the wave length of the operating frequency.

3. Structure in accordance with claim 1, including a second transistorconnected in emitter to emitter and base to base relation with respectto said first mentioned transistor, the followers of both transistorsbeing connected to a relay circuit, whereby a received Doppler signalmay be of push-pull form, thereby cancelling inherent noise from saidsignal.

1. In a self-mixing Doppler radar construction including a coaxialtransmission element and a rod antenna extending from a transmitting andreceiving cavity, the improvement comprising: said coaxial transmittingelement including a first outer coaxial shield, a cavity-forminghousing, means on a outer surface of said housing for mounting saidshield at substantially right angles with respect thereto, a microwaveoscillator transistor, a second tube coaxially disposed within saidouter shield interconnected to the base of said transistor, a third tubecoaxially disposed within said second tube and interconnected to thecollector of said transistor; said transistor being disposed within thebase of said third tube, and having an emitter lead extending into saidcavity in said housing.
 2. Structure in accordance with claim 1, furthercharacterized in that said third tube has a length, and said dielectriccavity has a diameter equal to one quarter of the wave length of theoperating frequency.
 3. Structure in accordance with claim 1, includinga second transistor connected in emitter to emitter and base to baserelation with respect to said first mentioned transistor, the followersof both transistors being connected to a relay circuit, whereby areceived Doppler signal may be of push-pull form, thereby cancellinginherent noise from said signal.